1. Field of the Invention
The present invention relates to a digital printing system and, in particular, to a digital printing system having an apparatus for controlling the velocity of a member.
2. Description of the Prior Art
To produce hard copy for digital printing, proofing or radiographic imaging systems, a photosensitive material is mounted on a substantially planar member known as a translation stage. The stage carries the photosensitive element rectilinearly along a relatively long travel path on the order of at least eight inches past a scanning laser beam. As the photosensitive element translates beneath a printing laser a beam of laser light is scanned in raster fashion across the element thereby recording the photographic or printed image onto the photosensitive element.
The scanned lines are relatively tightly spaced, with spacing between lines on the order of one hundred micrometers being not uncommon and with tolerances in that spacing being on the order of two micrometers. Accordingly, to avoid banding, the control of the motion of the stage must be made exceedingly exact. Banding is caused, for example, either by a redundancy of scan lines over the same area of the photosensitive element, the absence of a scan line or lines over some area of the element, or misplacement of scan lines. If banding occurs relatively long streaks extending in a direction perpendicular to the direction of motion of the stage member become obvious to the unaided eye. For certain types of imaging, such as in medical X-ray imagery, banded streaks cannot be tolerated since they interfere with the radiologist's ability to evaluate the radiogram. In other printing uses banding is objectionable due to its deleterious effect on the quality of the printed image.
Exempary of a velocity control arrangement for use in the environment of a digital printing system is that shown in U.S. Pat. No. 4,505,578 (Balasubramanian). This patent discloses a hydraulic braking arrangement for controlling the velocity of a stage member past a scanning head.
Velocity control schemes for use in connection with instruments performing high precision scientific research are known. For example, U.S. Pat. No. 3,488,123 (Nichols) relates to a velocity control scheme for controlling the velocity of a movable mirror of an interferometer used in experiments carried out in space craft. This device uses an electromechanical drive having a short travel distance, on the order of millimeters, and a radiation source with a limited coherence length. Such a control arrangement would not appear to be useful in the control of printing systems that require travel distances many times in excess of the travel path of the scientific instrument, particularly those using a less precisely controllable drive mechanism.
Accordingly, it is believed advantageous to provide a control arrangement for accurately controlling the velocity with which the translation stage is moved along a travel path at least eight inches in length.